Heat Exchangers in Aerospace Applications

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056

Volume: 08 Issue: 03 | Mar 2021 www.irjet.net p-ISSN: 2395-0072

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Heat Exchangers in Aerospace Applications

Narayan Thakur, Advait Inamdar, Deepak Pal, Akshat Mohite

Narayan Thakur, Mechanical Engineer, AP Shah Institute of Technology, Thane, Maharashtra, India Advait Inamdar, Mechanical Engineer, AP Shah Institute of Technology, Thane, Maharashtra, India

Deepak Pal, Mechanical Engineer, Rajendra Mane College of Engineering and Technology, Ratnagiri, Maharashtra, India

Akshat Mohite,Mechanical Engineer, AP Shah Institute of Technology, Thane, Maharashtra, India

---------------------------------------------------------------------***---------------------------------------------------------------------Abstract - Heat is a form of energy which is transferred

when there is a difference in temperature between two or

more bodies. In some cases transfer of heat energy can be

determined by simply applying the laws of thermodynamics

and fluid mechanics. Heat can be transferred by three

different means, namely, Conduction, Convection and

Radiation. The requirements of thermal management in

aerospace applications are continuously growing, whereas

the weight and volume requirements of the systems remain

constant or shrink. Aerospace systems have high heat flux

requirement and it requires compact, high performance and

light weight equipment with the capacity to withstand low

or no atmospheric pressure. Various experiments and

computational methods have been carried out for

management of heat transfer and its applications in an

aircraft. Although these experiments have not been able to

understand the basic mechanism of thermal convection and

thermal radiation in space applications. Heat exchangers

play a very important role in thermal management of an

aerospace system. This paper introduces numerous

standard applications of heat exchangers in aerospace

systems and presents a few concepts of heat exchangers,

which have been used in aerospace or may be thought about

as promising designs for aerospace industry.

Key Words: Heat, Heat energy, Thermodynamics, Fluid Mechanics, Conduction, Convection, Radiation, Thermal Management, Aerospace, heat flux, Heat exchangers.

1. INTRODUCTION Heat exchangers are equipment being used for transfer of

heat between two or more fluids at different

temperatures. They are widely used in power plants, auto

motives, space heating, refrigeration and air-conditioning

systems, aerospace industry, petrochemical processes and

electronics cooling. In aerospace industry, heat exchangers

are mainly used in three systems: (1) gas turbine cycle, (2)

environmental control system (ECS), and (3) thermal

management of power electronics. Heat exchangers may

be classified in various ways, e.g., based on transfer

processes, surface compactness, construction features,

flow arrangements, and heat transfer mechanisms. Design

and sizing of heat exchangers are generally complicated.

In general, heat transfer, pressure loss, cost, materials,

operating limits, size and weight are important

parameters. Especially for aerospace industry, the weight

and the size of the heat exchanger are most important.

Numerous improvement techniques, for example,

extended surfaces such as the fins, are used in heat

exchangers to enhance the surface area for heat transfer

or the heat transfer coefficient, or both. The aim of

improvement techniques could be to reduce the size of the

heat exchanger for a given function, to increase the

capacity of an existing heat exchanger, or to reduce the

approach temperature difference. Implementation of heat

transfer enhancement techniques in aerospace heat

exchangers might help reduce fuel consumption and the

size of the heat exchanger.

1. APPLICATIONS OF HEAT

EXCHANGERS IN AEROSPACE

2.1. GAS TURBINE CYCLES

Heat exchangers can be implemented in gas turbine cycles

to increase thermal efficiency. For a conventional gas

turbine cycle, the thermal efficiency mainly depends on

the overall pressure ratio and the turbine inlet

temperature. The overall pressure ratio indicates the ratio

of the compressor outlet pressure to the compressor inlet

pressure. The overall pressure ratio and the turbine inlet

temperature are limited by the maximum pressure and

temperature that the turbine blades can withstand. New

materials and innovative cooling concepts for the critical

components can further increase the overall pressure

ratio and the turbine inlet temperature.




A method to improve the overall pressure ratio for a given

compression work is to introduce multistage compression

with intercooling, in which the gas is compressed in stages

and cooled between each stage by passing the gas through

a heat exchanger called an intercooler. Gas turbine engines

in aerospace industry require high overall pressure ratio.

To achieve higher pressure ratio, the compressor is

divided into a low pressure compressor (LPC) and a high

pressure compressor (HPC). This is done to introduce an

intercooler between LPC and HPC. The compressed gas

has a relatively higher temperature at the outlet of LPC. By

using a crossflow or counterflow air-to-air heat exchanger,

with compressed air flowing on one side and low-

temperature ram air flowing on the other side, the

compressed air can be cooled before entering the HPC. The

steady-flow compression work or the pressure ratio for a

given compression work is proportional to the specific

volume of the compressed air [8]. The intercooler

decreases the temperature and hence decreases the

specific volume of the compressed air, which improves the

thermodynamic cycle efficiency. In gas turbine engines,

the temperature of the exhaust gases leaving the turbine is

often considerably higher than that of the air leaving the

HPC. A regenerator or a recuperator, i.e. a crossflow or

counterflow heat exchanger, can be incorporated to

transfer heat from the hot exhaust gases to the

compressed air. Accordingly, the thermal efficiency

increases because the portion of the energy of the exhaust

gases that is supposed to be rejected to the surroundings

is recovered to preheat the air entering the combustion

chamber. The recuperator is more advantageous when an

intercooler is used because a greater potential for

recuperation exists. A recuperator is not effective, for

highoverall pressure ratios, especially considering its cost,

size and weight. Fig 1 shows a conceptual sketch that

compares the thermal efficiencies of different gas turbine

cycles with the overall pressure ratios. In general, the

intercooled and recuperated gas turbine cycle is effective

at relatively low overall pressure ratios, e.g. less than 30.

The intercooled gas turbine cycle without recuperation is

only effective at very high overall pressure ratios. Fig 2

illustrates an intercooled and recuperated gas turbine

cycle.

Figure.1 Thermal efficiency versus overall pressure ratio

for different gas turbine cycles. Adapted from Sieber J,

Overview NEWAC (new aero engine core concepts); April

2009.

Figure.2 Intercooled and recuperated gas turbine cycle.

LPC, low-pressure compressor; HPC, high-pressure

compressor; Com, compressor; HPT, high-pressure

turbine; LPT, low-pressure turbine Adapted from Sieber J,

Overview NEWAC (new aero engine core concepts); April

2009.

In addition, precoolers, such as cryogenic fuel-cooled air

heat exchangers, that immediately downstream the air

intake to precool the air entering the engine can be used

for high-speed jet engines to save fuel consumption for a

given overall pressure ratio or to increase the overall

pressure ratio for a given compression work. Besides,

modern gas turbine engines operate at turbine inlet air

temperature levels that are beyond the material maximum

temperature. Hence, hot engine components such as the

turbine blades must be cooled to assure their structural

integrity. Typically the turbine blades are cooled by bleed

air from the compressor, which, although cooler than the

turbine, has been already heated up by the work done on

it by the compressor [8]. Engine bleed reduces thrust and

increases fuel burn. The effect is a function of the mass

flow rate of the bleed air. If a heat exchanger is used to




cool the bleed air before its introduction for blade cooling,

the amount of bleed air required by the blade cooling can

be reduced, which results in an improved engine

performance with a consequent reduction in specific fuel

consumption (the rate of fuel consumption per unit of

power output). Such a heat exchanger might be a fuel-to-

air heat exchanger. The relatively hot bleed air can be

cooled by the cool engine fuel. On one hand, the cooling

capacity of the bleed air increases as its temperature is

decreased. On the other hand, the energy extracted by the

fuel is reintroduced into the propulsive cycle as the heated

fuel is burned in the combustor.

1.2 ENVIRONMENTAL CONTROL SYSTEM

The ECS is employed in aerospace vehicles such as large

commercial aircraft to provide comfortable flight

conditions for passengers. There are two kinds of air-

conditioning systems on an aircraft: air cycle air-

conditioning and vapor cycle refrigeration system. For air

cycle air-conditioning, the source of air used by the ECS is

typically bleed air from the gas turbine compressor, with a

relatively high pressure and temperature. The hot bleed

air must first be cooled to an acceptable temperature

before entering into the passenger cabin. This is

performed using an air-conditioning package that is

composed of several units, including a number of heat

exchangers cooled by ambient ram air. As shown in Fig. 3

the hot bleed air from the engine compressor is metered

through a bleed air valve and then cooled by the primary

heat exchanger. Then the air passes to the air cycle

machine for pressure adjustment and finally into the

secondary heat exchanger for temperature adjustment

before entering the cabin. The primary and secondary heat

exchangers shown in Fig.3 are air-to-air heat exchangers.

A vapor cycle refrigeration system, in which the

refrigerant undergoes phase changes, is basically the same

refrigeration cycle that is used in home air conditioners.

The vapor cycle refrigeration system is a closed system

used for transferring heat from inside the cabin to outside

the cabin. Heat exchangers such as evaporators and

condensers are important devices in the refrigeration

cycle. Specifically the liquid refrigerant decreases its

pressure by passing through a thermal expansion valve

before entering the evaporator. The liquid refrigerant in

the evaporator changes into vapor, absorbing the heat

energy from the cabin air. The vapor is then compressed

and becomes hot. The hot vapor refrigerant transfers its

heat energy to the outside ram air in a condenser and thus

the refrigerant condenses back into liquid to repeat the

cycle. Therefore, the warm cabin air is constantly replaced

or mixed with cool air in the aircraft cabin to maintain a

comfortable temperature. An example of a vapor cycle

refrigeration system is shown in Fig. 4.

Figure 3 A schematic of an Airbus air-conditioning system

[10]. HEX, heat exchanger.

Figure 4 A vapor cycle refrigeration system.

1.3 THERMAL MANAGEMENT

The trend to pack current and future aerospace and

military platforms with power-hungry, heat-generating

electronic components and systems drives the need for

efficient, effective, compact, and lightweight thermal

management systems. Shrinking electronics packaging and

high-density integration of power electronics to enable

more power and functionality in a small unit, coupled with

the extremes of military and aerospace environments,

constantly place ever-increasing demands on precise and




smart thermal management solutions to maintain junction

temperatures below levels that degrade performance. The

temperature of the components can be maintained at a

safe level by air cooling for low-heat-flux components. As

the heat flux increases, the limits of air cooling technology

are being approached, i.e., forced air heat sinks have

become significantly larger, more expensive, and more

complex. Liquid cooling is promising to provide the

needed level of thermal performance, with an increase in

energy efficiency compared to traditional air cooling. The

liquid must pass through the heat sources to carry away

the heat, resulting in a temperature increase in the liquid.

Then the liquid passes through a liquid-to-air heat

exchanger, such as a plate-fin heat exchanger (PFHE), to

transfer the stored heat to the air. An important

component for liquid cooling is the cold plate. The cold

plate must be able to dissipate the waste heat efficiently

within a relatively small unit. Fig.5 shows examples of cold

plates, namely, the tube liquid cold plate and the

powdered metal cold plate [11], with the former suitable

for low heat fluxes and the latter for high heat fluxes. Cold

plates with embedded microchannels are promising to

dissipate high heat fluxes. In many cases, heat pipes are

embedded in cold plates to increase the effective thermal

conductivity of the cold plates. Basically the cold plates

should be designed by conforming to the heat-generating

components. Besides, the cold plates should be optimized

to augment the thermal performance while maintaining a

relatively low pressure drop penalty by properly

designing, e.g., the liquid flow passages and manifold

distributions inside the cold plates. A liquid cooling

concept is sketched in Fig.6.

Figure 5 Cold plate examples: (a) a tube liquid cold plate

and (b) a powdered metal cold plate.

For ultrahigh heat flux components such as converters or

densely packed computing devices with heat fluxes

greater than 100 W/cm2, liquid cooling might not be

sufficient enough to cool the heat-generating components

with certain form factors. Besides, similar to air cooling,

liquid cooling cannot provide relatively uniform cooling as

the liquid temperature increases along the cooling path.

One way of providing an efficient cooling system for high-

power-density electronic devices is by using an

evaporative cooling circuit. This circuit brings a liquid into

thermal contact with the heat-generating components via

an evaporating unit to effectively remove heat by utilizing

the latent heat of the working fluids and at the same time

to maintain a relatively uniform temperature distribution.

Compared to air cooling and single-phase liquid cooling,

microchannel evaporative cooling has a low pumping

power requirement relative to the quantity of heat to be

removed [12]. At the same total flow rates, the pressure

drop in evaporative microchannels is much higher than

that for single-phase liquid flow in conventional channels.

However, to dissipate the same amount of heat, much less

liquid is required in microchannel evaporative cooling

than that in single-phase liquid cooling. Fig.7 shows an

evaporative cooling loop with a pump, a cold plate

evaporator, an air-cooled condenser, and a reservoir. It

has been stated that such an evaporative cooling system

with refrigerant R134a as the working fluid can provide

twice the cooling capacity in half the size or in a size less

than that of air- or water-cooling systems [13].

Figure 6 A concept of a liquid cooling solution.

Besides, the temperature of lubricating oil needs to be

controlled. The excessive heat caused by friction in devices

such as gearboxes and compressors increases the

lubricant oil temperature. As the oil viscosity decreases

with increment in temperature, the lubricant oil film

becomes thinner and collapses, which in turn causes more

friction and heat. Fuel or ambient ram air can be used as

working fluids in fuel-cooled oil cooler or air-cooled oil

cooler to cool the lubricants.




Figure 7 An exemplified evaporative cooling system.

3. GENERAL DESIGN CONSIDERATIONS FOR

AEROSPACE HEAT EXCHANGERS

The typical features of aerospace heat exchangers are

briefly listed below.

• Compactness and lightweight: Compact heat exchangers

are promising because of the limited space available for

heat transfer in aerospace industry. Light materials such

as aluminum alloy and metal foam might be preferred if

the operating pressure and temperature are less than the

maximum allowable pressure and temperature of the

materials.

• Relatively high temperature and pressure for some

aerospace heat exchangers such as recuperators:

Compressed air leaving the HPC flows on one side of the

recuperator and the exhaust gas exits from the turbine

flows on the other side. For example, for an overall

pressure ratio of 50, the compressed air leaving the HPC

has a pressure of 50 bar and a temperature up to 1000 K.

The materials for high-temperature heat exchangers

should be able to maintain structural integrity to cope

with large temperature differences.

• High effectiveness and minimum pressure loss: The heat-

exchanging surfaces and flow paths should be designed in

such a way to give a high thermal conductance with a

minimum pressure loss. Besides, the bypass duct and the

inlet and outlet ducts to and from the heat exchangers

need to be arranged properly to minimize momentum

pressure losses.

4. TYPES OF HEAT EXCHANGERS

4.1 PLATE-FIN HEAT EXCHANGERS

A PFHE consists of flat plates and finned chambers to

transfer heat between fluids. PFHE is often categorized as

a compact heat exchanger with a relatively high heat

transfer surface area to volume ratio. In aerospace

industry, aluminum alloy PFHEs have been used for 50

years for their compactness and lightweight properties.

Most commonly this heat exchanger type is for gas-to-gas

heat exchange. Various fin geometries such as triangular,

rectangular, wavy, louvered, perforated, serrated, or the

so-called offset strip fins are used to separate the plates

and create the flow channels. The common fin thickness

ranges from 0.046 to 0.20 mm and the fin height ranges

from 2 to 20 mm.

Figure 9. Types of fin geometries: (a) rectangular fin, (b)

wavy fin, (c) offset strip fin, and (d) louvered fin.

4.2 PRINTED CIRCUIT HEAT EXCHANGERS

The concept of PCHEs was first invented in the early

1980s at the University of Sydney in Australia. It has been

commercially manufactured by Heatric Ltd in the United

Kingdom since 1985. A brief introduction of PCHE is given

in Reay et al. [4]. The PCHE derives its name from the

procedure used to manufacture the plates that form the

core of the heat exchanger; the fluid flow passages are

produced by chemical milling, a technique similar to that

used to manufacture printed circuit boards in the

electronics industry. A benefit of this design is the

flexibility it affords in terms of flow passage geometry. The

fluid flow passages for PCHE have approximately a

semicircular cross section. Channel depths and channel

widths are typically in the range 0.5–2.0 mm and 0.5–5.0

mm, respectively, and may be larger for some streams.

PCHEs are highly compact because of their high surface




area-to-volume ratios (>2500 m2/m3) [15]. A PCHE is a

high-efficiency plate-type compact heat exchanger, which

is typically four to six times smaller and lighter than a

shell-and-tube heat exchanger of equivalent performance.

Figure 10. A diffused PCHE

4.3 MICRO HEAT EXCHANGERS

Micro- and miniscale heat exchangers (μHEXs) are heat

exchangers in which at least one fluid flows in lateral

confinements such as channels or small cavities with

dimensions less than around 1 mm in size. Some PFHEs

and PCHEs can also be categorized as μHEXs. Typically, the

fluid flows through a cavity that is called a microchannel.

This process intensification technology exploits heat

transfer enhancement resulting from structurally

constrained fluid streams in microchannels, which reduces

the thermal resistance considerably. Micro heat

exchangers have been demonstrated with high heat

transfer coefficients, approximately one order of

magnitude higher than the typical values of conventional

heat exchangers. Therefore, as a typical miniaturized

process device, micro heat exchangers have attracted

widespread applications because of their high thermal

performance, compactness, small size, and lightweight

[20], with the aim of process intensification. Micro heat

exchangers are used in diverse energy and process

applications such as electronics cooling, automotive and

aerospace industries, chemical process intensification,

refrigeration and cryogenic systems, and fuel cells. Micro

heat exchangers have many advantages over conventional

heat exchangers. First, in the scaling down from macro- to

microscale, the volume decreases with the third power of

the characteristic linear dimensions, whereas the surface

area only decreases with the second power. Therefore,

micro heat exchangers have relatively larger surface area-

to-volume ratios that enable higher heat transfer rates

than conventional heat exchangers. Second, fast fluid

acceleration and close proximity of the bulk fluid to the

wall surface in μHEXs give high heat transfer coefficients.

For single-phase fully developed internal laminar flow, a

constant Nusselt number (Nu = hdh/k) implies that the

single-phase heat transfer coefficient increases as the

hydraulic diameter decreases. In addition, compactness

and high heat-flux dissipation are required as the scale of

the devices decreases, whereas the power density

increases. Micro heat exchangers are expected to manage

the high heat-flux dissipation for miniaturized devices to

guarantee the reliability and safety during operation.

Figure 11. Examples of micro heat exchangers: (a)

ceramic micro heat exchanger [24] and (b) 3D-printed

heat exchanger by Sustainable Engine Systems Ltd. [4].

5. OTHER AEROSPACE HEAT EXCHANGERS

5.1 PRIMARY SURFACE HEAT EXCHANGERS

Primary surface heat exchangers refer to heat exchangers

wherein substantially all the material that conduct heat

between two media comprises the walls separating the

two media. This type of heat exchangers primarily consists

of plates or sheets and have no separate or additional

internal members, such as fins, so that the exchanger is

constructed of plates or sheets, each side of which is in

contact with a different fluid, and heat transfer occurs

substantially and directly between the plates and the fluid.

In contrast, extended surface heat exchangers, or

secondary surface heat exchangers, contain a substantial




amount of material in the form of fins, which do not

separate the media. The PFHE and micro heat exchanger

are mostly extended surface heat exchangers. The main

attributes of primary surface heat exchangers are that the

surface geometry is 100% effective (no fins) and sealing

can be accomplished by welding without the need for an

expensive and time-consuming high-temperature furnace

brazing operation. Primary surface heat exchangers can be

used as intercoolers and recuperators in the aerospace

industry.

5.2 HEAT PIPE HEAT EXCHANGER

Heat pipes of various capillary wick structures are

attractive in the area of spacecraft cooling and

temperature stabilization because of their lightweight,

zero maintenance, and reliability. A heat pipe consists of

an evaporation zone, an adiabatic zone, and a

condensation zone. Liquid in contact with the evaporation

zone turns into vapor by absorbing heat, and vapor then

travels along the heat pipe to the condensation zone and

condenses back into liquid, releasing the latent heat. The

liquid then returns to the evaporation zone through

capillary forces or other external forces. Loop heat pipes

and micro/miniature heat pipes are promising types

suitable for cooling solutions. Micro heat pipes are heat

pipes with a height less than about 1 mm. Micro and

miniature heat pipes are able to dissipate high heat fluxes

up to 100 W/cm2 [29]. A major advantage of heat pipes is

that no external power is required. For high heat fluxes,

heat pipes need a relatively larger area for heat dissipation

from the heat pipe condensation zone to the environment,

which might be a concern because of the limited space in

aerospace. The depth of the micro heat pipe is less than

1.0 mm. The width and the length of the evaporation zone

depend on the form factor of the electronic components.

The length of the condensation zone can be designed by

the required dissipated heat and the cooling mode of the

external heat sink.

Figure 12. A flat micro heat pipe.

6. HEAT EXCHANGERS USING NEW MATERIALS

6.1 FOAM MATERIALS

Light materials are preferred in the manufacturing of

aerospace heat exchangers to save fuel consumption.

Aluminum alloy heat exchangers such as aluminum alloy

PFHEs are common in the aerospace industry. Foam

materials such as open-cell porous metallic foams and

graphite foams have received increased attention for use

in commercial heat exchangers because of their

lightweight, improved thermal performance, high

compactness, attractive flexibility to be formed in complex

shapes, good stiffness/strength properties, and low cost

via the metal sintering route for mass production, as well

as because some materials can be used at high

temperatures up to 1200 K. Because of the

interconnection of pores and tortuosity of open-cell foams,

fluid mixing and convection heat transfer are greatly

enhanced. The key parameters that influence the

thermohydraulic performance of foams are porosity, pore

density (pores per inch), mean pore diameter, surface

area-to-volume ratio, effective thermal conductivity, and

permeability. The pertinent correlations for flow and

thermal transport in metal foam heat exchangers were

categorized in Ref. [30] Graphite foam is attractive, as the

density of graphite foam ranges from 200 to 600 kg/m3,

which is about 20% of that of aluminum. However, the

tensile strength of graphite foam is much less than that of

metal foam. The weak mechanical properties of graphite

foam block its development as a heat exchanger. Adding

more material into the graphite foam or changing the

fabrication process might improve the mechanical

properties of the foam [31]. The major disadvantage of

open-cell foams is the relatively high pressure drop

penalty, as pointed out by Muley et al. [32]. Besides, at

present, foam heat exchangers are not suitable for high-

pressure conditions, as the maximum pressure for foam

heat exchangers is relatively low.

6.2 CERAMIC MATERIALS

The two main advantages of using ceramic materials and

ceramic matrix composites (CMCs) in heat exchanger

construction over traditional metallic materials are their

temperature resistance and corrosion resistance. Ceramics

and CMCs are now used in rocket nozzles and jet engines,

heat shields for space vehicles, aircraft brakes, gas

turbines for power plants, fusion reactor walls, heat

treatment furnaces, heat recovery systems, etc. Major




obstacles preventing the wide use of ceramics include

ceramic-metallic mechanical sealing, manufacturing costs

and methods, and their brittleness in tension. The most

commonly used CMCs are non-oxide CMCs, i.e.,

carbon/carbon, carbon/silicon carbide, and silicon

carbide/silicon carbide. At present, various heat

exchangers, such as PFHEs, micro heat exchangers, and

primary surface heat exchangers, can be manufactured by

ceramics. Ceramic recuperators and intercoolers can be

used to improve the gas turbine cycle efficiency.

Figure 13. (a) Open-cell metal foam and (b) a foam heat exchanger [32].

7. CONCLUSION

Aerospace heat exchangers are mainly used in gas turbine

cycles, ECSs, and thermal management of various

aerospace systems. The applications of aerospace heat

exchangers are discussed. The main features are listed for

aerospace heat exchangers. This paper also illustrates

various typical compact heat exchangers including PFHEs,

micro heat exchangers, PCHEs, primary surface heat

exchangers, and heat pipes, which are favorable to provide

efficient heat transfer and cooling in the aerospace

industry.

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Documents

Heat Exchangers in Aerospace Applications